Mobile communication systems include portable handheld RF communication devices such as cellular telephones, laptop computers, and data communication devices (PDAs) as well as infrastructure devices such as base stations, controllers and the like. Such systems typically operate according to a set of industry standards or protocols in one or more bands. One example of communication devices whose popularity is continually growing is Push-to-talk (PTT) radios, which previously have been used primarily by emergency personnel. PTT radios generally enable a user to communicate with a group of one or more users (talkgroup) on the same channel.
One popular band used by PTT radios is the 900 MHz Industrial, Scientific and Medical (ISM) band. Frequency-hopping spread spectrum communication is employed in the ISM band to reduce the susceptibility of radios to interference and maximize communication range. However, many countries have regulations regarding spectrum usage. In the United States, for example, the frequency hopping spectral distributions are governed by the Federal Communication Commission (FCC) and must meet distribution requirements that utilize spectrum uniformly across the ISM band.
In general, there are three phases of a call: establishing a connection in a connection establishment phase, transmitting data in a traffic phase, and terminating the connection in a stop phase. Each of these phases involves sending control or data information from the originator to the target and perhaps acknowledging the receipt of the transmission during different frames of a predetermined duration. The traffic phase contains TCH (Traffic CHannel) or simply traffic frames and the stop phase contains a SSW frame.
In the connection establishment phase, the target determines whether a call is present and if so the manner in which to adjust its frequency to receive data during the traffic phase of the call. During this phase, the target receives control information such as synchronization in various frames of information. While the use of spread spectrum techniques provides advantages, these techniques rely on radios receiving the call setup information during the connection establishment phase to enable data to be received during the traffic phase. If a target misses some or all of the call setup information, a target may not be able to join a call that is in progress (also referred to as an on-going call). Instead, the target waits for the current call to end and for the next call to start. Moreover, if the target loses communication with the originator by dropping a sufficient number of consecutive packets, it is unable to re-join the call.
It is thus desirable to provide a means for a target to join a call that is already in progress.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments shown so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Other elements, such as those known to one of skill in the art, may thus be present.